The present invention relates to an exhaust gas deNOx apparatus for an engine, and particularly, it relates to an in exhaust gas deNOx apparatus for a diesel engine.
Various contrivances are conventionally proposed regarding an exhaust gas deNOx apparatus for a diesel engine, and one disclosed in Japanese Patent Application Laid-open No. 7-279718 is cited as an example thereof.
According, to Japanese Patent Application Laid-open No. 7-279718, an NOx adsorber is placed in an exhaust passage of an engine, and an exhaust gas recirculating (EGR) circuit having an EGR control valve for controlling an amount of flowing exhaust is provided. The NOx adsorber absorbs NOx in a normal condition, and it releases NOx when an air fuel ratio inside a combustion chamber becomes rich, and reduces NOx by CO and UC. When the air fuel ratio inside the combustion chamber is to be made rich, the EGR control valve is opened to allow exhaust gas to recirculate, an amount of intake air is decreased, and injection fuel is increased to thereby make the air fuel ratio rich. At the same time, a fuel injection amount is further increased so that engine output torque does not change.
However, in the above-described constitution, a large amount of NOx is contained in exhaust gas because the EGR control valve is closed during normal operation, and therefore it is necessary to increase the size of the NOx adsorber, which makes a space area larger in the case of a large-sized engine, thus causing the constitutional disadvantage, and also causing the disadvantage of cost being high. In addition, when fuel is increased under a heavy load, there arises the fear that inner pressure of cylinders and exhaust gas temperature are increased and reliability and durability of the engine are impaired.
The present invention is made in view of the above-described disadvantages, and its object is to provide an exhaust gas deNOx apparatus for an engine at low cost, which can reduce an NOx adsorber catalyst in size without impairing reliability and durability of an engine.
In order to attain the above object, an exhaust gas deNOx apparatus for an engine according to the present invention is an exhaust gas deNOx apparatus for an engine comprising
an NOx adsorber catalyst in an exhaust pipe line of an engine, for adsorbing NOx when an air fuel ratio of an exhaust gas flowing therein is in a lean state and releasing NOx when the air fuel ratio of the exhaust gas flowing therein is in a rich state, and
an exhaust gas recirculaing circuit for mixing the exhaust gas into intake air, and has the constitution provided with exhaust gas recirculating amount control means for recirculating a predetermined amount of exhaust gas for reducing NOx when an adsorbed NOx accumulation amount is not more than a predetermined value, and recirculating the predetermined amount or more of exhaust gas to bring the air fuel ratio into a rich state when the adsorbed NOx accumulation amount exceeds the predetermined value and the NOx is to be released.
According to the above constitution, a predetermined amount (for example, 10% to 15% of the exhaust gas amount) of exhaust gas can be recirculated when the air fuel ratio of the exhaust gas of the engine is in a lean state, and therefore NOx in the exhaust gas can be reduced in a normal lean-burn state. In addition, since the exhaust gas recirculating amount control means is provided, the exhaust gas recirculating amount can be adjusted to the amount that can optimize emission, and the NOx adsorber catalyst can be made compact, thus reducing a space area to make the engine compact.
Further, in the exhaust gas deNOx apparatus for the engine,
the exhaust gas recirculating amount control means has the constitution in which it is able to adjust an exhaust gas recirculating amount, which is fed when the air fuel ratio of the exhaust gas flowing into the NOx adsorber catalyst is brought into a rich state, so that an excess air ratio is more than 1.0 and is about 1.3 or less.
According to the above constitution, the exhaust gas recirculating amount control means that can make the exhaust gas recirculating amount have the excess air ratio of 1.0 to about 1.3 is provided. As a result, the air fuel ratio of the exhaust gas is brought into a rich state, whereby NOx can be released and reduced, a large amount of fuel as a reducer does not have to be sprayed, and fuel consumption rate can be prevented from changing for the worse. The excess air ratio is xe2x80x9c1xe2x80x9d when the air fuel ratio is a theoretical mixture ratio. The rich state means the state in which the air fuel ratio is near the theoretical mixture ratio, and the lean state corresponds to the state in which the air fuel ratio is larger than in the rich state, but the excess air ratio corresponding to each state differs according to the kind of the engine. When the above-described exhaust gas recirculating amount is to be adjusted, the oxygen concentration in the exhaust gas is converted into air with the oxygen concentration of about 20%, the converted air is regarded as part of the intake air amount and fresh air is added thereto, which is made the total air amount.
Further, in the exhaust gas deNOx apparatus for the engine,
the exhaust gas recirculating amount control means comprises
at least any one of an intake air throttle valve free to be opened and closed, which is provided in a pipe line of an intake pipe of the engine, and an exhaust gas throttle valve free to be opened and closed, which is provided in a pipe line of an exhaust pipe of the engine, and
a controller, and
the controller outputs a control signal to decrease degree of opening of either one of the intake air throttle valve or the exhaust gas throttle valve when the air fuel ratio of the exhaust gas is to be brought into a rich state.
According to the above constitution, by decreasing the degree of opening of the throttle valve provided in the exhaust gas pipe line or the intake air pipe line, the supply air pressure can be easily lowered more sharply than the exhaust gas pressure. As a result, a large amount of exhaust gas is recirculated and thereby the air fuel ratio of the exhaust gas can be made rich, which makes it possible to reduce a space area and cost with the simple structure.
Further, in the exhaust gas deNOx apparatus for the engine,
the exhaust gas recirculating circuit comprises a first exhaust gas recirculating circuit and at least one second exhaust gas recirculating circuit provided in parallel with the first exhaust gas recirculating circuit,
the exhaust gas recirculating amount control means comprises a second recirculating circuit adjusting valve free to be opened and closed, which is provided in the second exhaust gas recirculating circuit,
NOx amount detection means for detecting an NOx amount, and
a controller for calculating an NOx accumulation amount in the NOx adsorber catalyst based on the NOx amount outputted from the NOx amount detection means and determining whether the calculated NOx accumulation amount is a predetermined value or less, or not, and
when the calculated NOx accumulation amount exceeds the predetermined value, the controller outputs a control signal to open the second recirculating circuit adjusting valve to make the air fuel ratio of the exhaust gas of the engine rich.
According to the above constitution, the exhaust gas is recirculated only in the first exhaust gas recirculating circuit when the air fuel ratio is lean. On the other hand, when the air fuel ratio is made rich, the second exhaust gas recirculating circuit is also used, and therefore a large amount of exhaust gas can be recirculated with less resistance. As a result, the performance is improved, and the second recirculating circuit adjusting valve is made an ON-OFF valve with a simple structure, thus making it possible to reduce cost.
In the exhaust gas deNOx apparatus for the engine, the exhaust gas recirculating circuit comprises a third exhaust gas recirculating circuit provided with a plurality of recirculating circuit adjusting valves in parallel, the exhaust gas recirculating amount control means comprises the plurality of recirculating circuit adjusting valves, NOx amount detection means for detecting an NOx amount, and
a controller for calculating an NOx accumulation amount in the NOx adsorber catalyst based on the NOx amount outputted from the NOx amount detection means and determining whether the calculated NOx accumulation amount is a predetermined value or less, or not, and
when the calculated NOx accumulation amount exceeds a predetermined value, the controller outputs a control signal to increase a total opening area of the plurality of recirculating circuit adjusting valves to be larger than that in a lean state and makes the air fuel ratio of the exhaust gas of the engine rich.
According to the above constitution, with only one exhaust gas recirculating circuit, fine adjustment is facilitated by making the first recirculating circuit adjusting valve compact when the air fuel ratio is lean, and thus emission can be optimized. When the air fuel ratio is to be made rich, the second recirculating circuit adjusting valve is opened to make the total opening area larger, whereby a large amount of exhaust gas can be recirculated. In addition, an ON-OFF valve can be used as the second recirculating circuit adjusting valve, which is simple in structure, and the engine can be constituted to be compact, which can make the cost lower.
Further, in the exhaust gas deNOx apparatus for the engine,
a turbocharger is provided at the engine, and
the exhaust gas recirculating amount control means comprises
an air bleed valve free to be opened and closed, which is provided at an outlet port of a compressor of the turbocharger,
and extracts supply air to an outside, and
a controller for outputting a control signal to open the air bleed valve when the air fuel ratio of the exhaust gas of the engine is to be made rich.
According to the above constitution, by opening the air bleed valve provided at the outlet port of the compressor of the turbocharger, the supply air pressure can be more sufficiently reduced than the exhaust gas pressure. Accordingly, a large amount of exhaust gas can be recirculated.
Furthermore, in the exhaust deNOx apparatus for the engine,
the exhaust gas recirculating amount control means comprises fuel supply means for supplying fuel into at least one of cylinders of the engine and an exhaust pipe of the engine, when the air fuel ratio of the exhaust gas is brought into a rich state, the fuel supply means supplies fuel as an adjustment amount to provide an air fuel ratio that allows the NOx adsorber catalyst to release and reduce NOx.
According to the above constitution, when the air fuel ratio is to be made rich, fuel is supplied to adjust an insufficient amount. As a result, the air fuel ratio can be surely controlled to be near the theoretical air fuel ratio. Further, since a large amount of recirculating exhaust gas is circulated, supply amount of fuel can be reduced, which is economical and can prevent the inner pressure of the cylinders and exhaust gas temperature from excessively rising, and thus the air fuel ratio can be made rich without reducing reliability and durability of the engine.
In the exhaust gas deNOx apparatus for the engine,
the exhaust gas recirculating amount control means includes load detection means for detecting a load of the engine, and a controller, and
the controller inputs a detection signal from the load detection means, and when the detected load is a predetermined value or less, it brings the air fuel ratio of the exhaust gas into a rich state.
According to the above constitution, when the engine is operated under a lighter load than a predetermined load, that is, in a light load range, the means for making the air fuel ratio rich is operated. Since the engine is under a light load, added fuel can be reduced, which is economical and can prevent the inner pressure of the cylinders and the exhaust gas temperature from excessively rising and the like, and the frequency of addition of fuel in a high load range is decreased, thus making it possible to improve reliability and durability of the engine.
Further, in the exhaust gas deNOx apparatus for the engine,
the exhaust gas recirculating amount control means comprises
a variable turbocharger provided at the engine, for making a degree of opening of a turbine passage variable, and a
controller, and
when the air fuel ratio is to be brought into a rich state, the controller outputs a control signal to decrease the degree of opening of the turbine passage.
According to the above constitution, the degree of opening of the turbine passage is made small, for example, the degree of opening is made extremely small, whereby the exhaust gas pressure at the exhaust gas turbine inlet port can be made higher than the supply air pressure at the compressor outlet port of the variable turbocharger, Then, this pressure difference between the supply air and exhaust gas is utilized to allow a required amount of exhaust gas to flow into the supply air. Accordingly, with use of the variable turbocharger, the same effects as the exhaust gas throttle valve can be also obtained.